The heart of a computer is a non-volatile storage device that is referred to as a magnetic disk drive. The magnetic disk drive includes a magnetic disk, and write and read heads. The write and read heads are supported by a slider that is mounted on a suspension arm. When the magnetic disk rotates, an actuator swings the suspension arm to place the write and read heads over selected circular tracks on the surface of the rotating magnetic disk. An air flow generated by the rotation of the magnetic disk causes an air bearing surface (ABS) of the slider to fly at a very low elevation (fly height) over the surface of the rotating magnetic disk. The write and read heads write magnetic transitions to and read magnetic transitions from the rotating magnetic disk, respectively. Processing circuitry connected to the write and read heads then operates according to a computer program to implement writing and reading functions.
The write head includes a coil embedded in an insulation stack that is sandwiched between main and auxiliary poles. The main and auxiliary poles are magnetically coupled at a back gap and are coated with an overcoat. A write current conducted to the coil induces a magnetic flux in the main pole that causes a magnetic field to write the aforementioned magnetic impressions to the rotating magnetic disk.
The read head includes a CPP TMR or GMR sensor electrically connected with top and bottom electrodes, but electrically insulated by insulating films from bias stacks at two side regions. A sense current conducted through the top electrode, CPP TMR or GMR sensor, and bottom electrode allows changes of TMR or GMR resistance to be read in response to external magnetic fields of magnetic transitions from the rotating magnetic disk.
The CPP TMR sensor typically comprises an AlOx, TiOx or MgOx barrier layer, while the CPP GMR sensor typically comprises a Cu spacer layer wherein some O is incorporated. After annealing at 360° C. or higher, the MgOx TMR sensor exhibits a TMR coefficient much higher than the AlOx and TiOx TMR sensors, and thus appears to be the most attractive for the use in the read head. A currently used MgOx TMR sensor includes a 1 nm thick nonmagnetic MgOx barrier layer overlying a lower structure and underlying an upper structure. The lower structure comprises a 3 nm thick nonmagnetic Ta adhesion layer, a 15 nm thick antiferromagnetic 46% Pt-54% Mn (all compositions are given in atomic percent) alloy pinning layer, a 2.5 nm thick ferromagnetic 70% Co-30% Fe alloy keeper layer, a 0.8 nm thick nonmagnetic Ru spacer layer, a 3 nm thick ferromagnetic 60% Co-20% Fe-20% B alloy reference layer. The upper structure comprises a 3 nm ferromagnetic 60% Co-20%-Fe-20% B alloy sense layer and an 8 nm thick nonmagnetic Ta cap layer.
To ensure proper sensor operation, magnetizations of the Co—Fe alloy keeper and Co—Fe—B alloy reference layers must be rigidly pinned in opposite transverse directions perpendicular to an air bearing surface (ABS), while the magnetization of the sense layer is preferably oriented in a longitudinal direction parallel to the ABS after applying a sense current. This rigid pinning is achieved through a high unidirectional anisotropy field (HUA) induced by exchange coupling between the antiferromagnetic Pt—Mn alloy pinning and ferromagnetic Co—Fe alloy keeper layers in a transverse direction, and through a high antiparallel coupling field (HAP) induced by antiparallel coupling between the ferromagnetic Co—Fe alloy keeper and Co—Fe—B alloy reference layers across the Ru spacer layer in two opposite transverse directions. To achieve optimal TMR responses, a demagnetizing field (HD) induced by the net magnetic moment of the Co—Fe alloy keeper and Co—Fe alloy reference layers must balance with a ferromagnetic coupling field (HF) across the MgOx barrier layer. This field balance is preferably achieved with low HD and HF to minimize non-uniform field distributions particularly at sensor edges.
In order to improve thermal stability caused by unwanted hysteretic magnetization rotations of the Co—Fe alloy keeper and Co—Fe—B alloy reference layer, the MgOx TMR sensor has recently been modified by adding a 2 nm thick Ru seed layer on top of the Ta adhesion layer and replacing the Pt—Mn alloy pinning layer with a 6 nm thick antiferromagnetic 22% Ir-78% Mn alloy pinning layer. The Ru seed layer is not needed in the Pt—Mn alloy TMR sensor because the Pt—Mn alloy pinning layer only needs annealing for developing its antiferromagnetism through a transformation from a nonmagnetic face-centered-cubic (fcc) phase to an antiferromagnetic face-centered-tetragonal (fct) phase; and, the crystalline planes of the Pt—Mn alloy are randomly oriented on the Ta adhesion layer. However, the Ru seed layer is needed in the Ir—Mn alloy sensor since the Ru seed layer facilitates fcc {111} crystalline planes of the Ir—Mn alloy pinning layer to lie in parallel to the interface between the Ru seed layer and the Ir—Mn alloy pinning layer for developing antiferromagnetism in the Ir—Mn alloy pinning layer. As a result, exchange coupling between the Ir—Mn alloy pinning and Co—Fe alloy keeper layers induces HUA much higher, and an easy axis coercivity HCE much lower, than that between the Pt—Mn alloy pinning and Co—Fe alloy keeper layers. The high HUA ensures rigid pinning, while the low HCE minimizes unwanted hysteretic magnetization rotations.
However, in contrast to the Pt—Mn alloy pinning layer exhibiting a smooth surface topography, the Ir—Mn alloy pinning layer exhibits a rough surface topography inevitably induced by its strong fcc {111} crystalline texture. This rough surface topography causes the lower structure to grow with rough interfaces and the MgOx barrier layer to exhibit a wavy profile. As a result, with a deteriorated TMR effect, the Ir—Mn alloy TMR sensor exhibits a higher HF and a lower TMR coefficient (ΔRT/RJ, where RT is a tunneling resistance and RJ is a junction resistance) than the Pt—Mn alloy TMR sensor.
A plasma treatment has been applied to all the layers in the lower structure for smoothing interfaces. However, this smoothing technique has the following drawbacks: it deteriorates microstructural effects when applied to the surface of the Ru seed layer; interrupts the growth of the preferred crystalline texture when applied to the Ir—Mn alloy pinning layer; destroys the desired unidirectional anisotropy when applied to the interface at the Ir—Mn alloy pinning layer; reduces the strength of antiparallel coupling when applied to surfaces of the Co—Fe alloy keeper and Ru spacer layers; and, diminishes tunneling effects when applied to the surface of the Co—Fe—B alloy reference layer. In other words, this smoothing technique is not feasible.
Therefore, there is a strong felt need for a sensor structure, especially a TMR structure, that can provide a smooth, planar surface for the barrier layer, while at the same time exhibiting strong exchange coupling with the AFM layer and maintaining a high TMR coefficient.